Low attenuation optical fibers are desirable for many applications, including signal transmission over long distances. To achieve waveguiding, the optical fiber requires a high index core and a low index cladding with an adequate core-cladding index differential. Most optical fibers incorporate germania (GeO2) as an updopant (index-increasing dopant) in silica for the core and use undoped silica for the cladding. Fluctuations in the concentration of germania, however, lead to high attenuation due to Rayleigh scattering and limit the use of germania-doped fibers in low loss applications.
An alternative approach is to design the fiber with an undoped silica core and to include a downdopant in the silica cladding to achieve the core-cladding index differential needed for effective waveguiding. The most common downdopant for silica is fluorine. This approach suffers from two drawbacks. First, undoped silica has a high melt viscosity and produces a core having a high fictive temperature upon cooling of the melt at practical rates. The high fictive temperature is indicative of an unrelaxed structural state of the core silica glass and increases fiber attenuation through Rayleigh scattering. Second, doping a silica cladding with fluorine lowers the melt viscosity of the cladding. In order to achieve the core-cladding differential needed for effective waveguiding with an undoped silica core, however, the fluorine doping concentration in the cladding needs to be high. Although high fluorine concentrations can be achieved, incorporation of fluorine as a dopant at the necessary concentration leads to a significant reduction in the melt viscosity of the cladding. As a result, a large viscosity mismatch develops between the core and cladding regions during draw. The large viscosity mismatch leads to significant stresses in the core during cool down and is responsible for a stress-optic effect that lowers the index of the core, thus compromising the waveguiding characteristics of the fiber by reducing confinement. Alleviation of core stresses and stress optic effects requires drawing of fibers at speeds slow enough to relax stresses and equilibrate the structure of the fiber. The necessary speeds, however, are too slow for practical manufacturing.
There remains a need for fibers having low attenuation that can be manufactured at high speeds.